In the packaging industry, a layered sheet or film having a single barrier layer of, for example, ethylene vinyl alcohol polymer (hereinafter called "ethylene vinyl alcohol") or polyvinylidene chloride polymer (hereinafter called "Polyvinylidene chloride"), is widely used. A barrier layer serves to prolong the freshness and shelf-life of food by, for instance, acting as a barrier to oxygen and moisture, and retaining aroma. However, any imperfections in a barrier layer, will interrupt the integrity of the barrier. Therefore, there is a need for an improved packaging material, in particular an improved food packaging material.
In forming containers from coextruded sheet, considerable scrap is produced. For economic reasons, it is desirable to recycle the scrap. However, organoleptic contamination should be prevented, particularly for a food packaging material made from coextruded sheet. Therefore, there is a need for a food packaging material made from coextruded sheet, that includes a high percentage of scrap, say about 30-65% of the total volume, and yet protects a food product and consumer from organoleptic contamination by the recycled scrap.
Typically, the barrier layer of a laminated sheet is located at or near the center of the sheet. Accordingly, when the barrier layer is ethylene vinyl alcohol, which typically will have a higher softening point and a more rapid rate of crystallization than other polymers used in the laminate, the draw ratio is limited, and it is difficult to solid phase pressure form the laminated sheet without exceeding the desired forming temperatures of the other polymers. Therefore, a laminated sheet including an ethylene vinyl alcohol barrier layer and having an improved draw ratio and forming range, is needed.
As illustrated by U.S. Pat. No. 4,197,069 to Cloeren, a coextrusion apparatus having an adjustable vane blade between flow channels, is known. The adjustable blade provides for stream convergence at substantially equal flow velocities.
As exemplified by U.S. Pat. Nos. 3,397,428 to Donald, 3,479,425 to Lefevre et al, and 3,860,372 to Newman, Jr., encapsulation of a core stream is known. Japanese Patent Document No. 55/28825 illustrates a multimanifold die that, as shown in FIG. 7 thereof, is able to produce a core layer sandwiched within adjacent layers.
Also known is a coextrusion apparatus that includes a removable die mounted within a feedblock. The die includes a streamlined flow passage highly suited for barrier materials for which residence time should be limited.
Transverse flow occurs in the die, which is rigidly mounted between a first flow channel and a second flow channel. The die has an exit slot of less width than the width of these flow channels. As a result, a stream of a barrier material exiting from the die, is sandwiched within streams from these flow channels.
Pivotably mounted between the first flow channel and a third flow channel is an adjustable vane blade. Likewise, between the second flow channel and a fourth flow channel is another pivotably mounted, adjustable vane blade.
Each flow channel includes a tapered flow-restriction channel. As in U.S. Pat. No. 4,197,069, by adjustment of flow-restriction channel width through radial movement of a vane blade point end, the convergence of streams at substantially equal flow velocities is effected.
Adjacent to the point end of a vane blade is a removably mounted, adjustable distribution pin. The pin is externally accessible for adjustment. Cooperation of a distribution pin with the adjacent vane blade tip, provides, if needed, for profiling.
Profiling results in each layer of a laminate product having a substantially uniform widthwise cross-section. In the profiling process, the cross-sectional configuration of a stream, viewed perpendicular to the flow-direction, is transformed from a rectangular shape to a shape that is changed back to the original rectangular shape as a melt-laminate formed from converging streams, passes through a downstream die manifold.
Despite the improvements provided by these apparatus, none is suited for the coextrusion of a laminate containing more than one barrier layer. Therefore, there is a need for a process for coextruding a laminate containing more than one barrier layer.
An additional drawback of the removable die-containing coextrusion apparatus, is that the presence of streams on both sides of an adjustable vane blade, affords a complex flow control situation. Furthermore, situations will arise where the streams on each side of a vane blade have opposing needs. It is desirable that a coextrusion apparatus provide simplified and improved flow control.